An equalizer is an electronic device that attempts to reverse distortion incurred when a signal is transmitted through a channel. In digital communications, an equalizer promotes reduction of inter-symbol interference. Distortion occurs because a channel's frequency response is not perfectly linear. If an equalizer's transfer function is the inverse of the channel's transfer function within the frequency range of the transmitted information, the equalizer can provide a flat response. The transfer function of an equalizer is commonly defined by circuitry having a number of nodes or taps that correspond to coefficients of the transfer function. Each tap has one or more controls, such as gain and dither. Setting the gain, dither or other tap controls establishes the transfer function.
In some equalizers, the tap controls or settings are fixed or hard-wired into the circuitry. In other equalizers, a user can program these settings. In still other equalizers, known as adaptive equalizers, tap control signals are generated in response to inputs that can vary. For example, an adaptive equalizer can vary the tap control signals in response to mean squared error (MSE) in a feedback-based manner. MSE is a measure of the difference between the transmitted data and the equalizer output (i.e., the equalized signal). An adaptive equalizer attempts to determine the set or group of tap settings that results in the least MSE, i.e., minimization of the MSE. Some adaptive equalizers perform such a process during a training sequence in preparation for data transmission. Other adaptive equalizers perform such a process dynamically during the data transmission. The latter type of adaptive equalizer may perform an iterative process that converges to a solution that minimizes the MSE. Adaptive equalizer circuitry can include Decision Feedback Equalizer (DFE) circuitry, Feedforward Equalizer (FFE) circuitry, continuous-time linear equalizer (CTLE) circuitry, or a combination of one or more of these. A problem that sometimes occurs is that the solution converges to a local minimum MSE and is unable to find a more global or overall minimum that exists.